In the past fifteen years there has been more and more evidence that a healthy ear not only detects sound but also generates sounds. This "reverse energy flow" can be evoked by either a tone, multiple tones, or clicks. In addition, many ears also generate sounds in the absence of any deliberate stimulation; that is sound is generated spontaneously. In all cases reverse-energy-flow originates in the cochlea, travels through the middle-ear and can be measured as otoacoustic emissions in the ear canal. The accessibility of otoacoustic emissions to measurements has made them a useful tool for objective evaluation of cochlear function. However, little is known about the effect of the middle-ear on otoacoustic emissions; for instance how are these emissions affected by middle-ear pathology. This project seeks to investigate the effect of the middle ear in reverse transmission through normal and pathological middle ears. A human temporal bone preparation will be used to directly measure reverse transmission. Sound will be generated in the inner-ear by coupling a custom built miniature hydro sound source into the vestibule. The resulting inner-ear sound pressure will be measured with a miniature hydrophone and the ear canal pressure measured with a sensitive probe-tube microphone. The ratio of the two pressures is the reverse pressure transfer function. These will be the first measurements of backward transmission in a human ear. The middle-ear input impedance and forward pressure transfer function will also be measured in the same preparations. The forward and reverse transmission measurements made before and after specific surgical modifications of the middle-ear - for example puncture of the tympanic membrane, interruption of the ossicles, and fixation of the ossicles - will both (1) empirically describe the effect of certain middle-ear pathologies and (2) enable a precise mathematical model description of the function of the middle ear. The model and measurements will be used to predict the effect of middle-ear pathology on ear canal otoacoustic emission measurements.